Add defensive check if the list is available in the morphology data before retrieving it

[FAMarfuaty]

Context

• Premium 23.9 uses English morphology data version 5, while Free 24.0 requires morphology data version 6, where a new list (with a new key) was added. This list is used in the Free 24.0. Hence, since Free 24.0 is trying to use a list that morphology data version 5 doesn't have, the assessments were broken.
• The solution is adding a defensive coding before retrieving the new list.

Summary

This PR can be summarized in the following changelog entry:

• Fixes an unreleased bug where the keyphrase assessments were broken when Yoast SEO 24.0 RC was paired with an older version of the premium add-on.

Relevant technical choices:

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

• Install and activate Free SEO 24.0-RC2 + Premium SEO 23.9
  • If building the plugin, you can pair this free branch with the wordpress-seo-premium main branch
• Create a new post
• Add keyphrase and text
• Check the console, and confirm that you don't get TypeError: Cannot read properties of undefined (reading 'list') (check the screenshot to see how it was before)
• Check Yoast metabox and sidebar, and confirm that assessments are loaded without error (check the screenshot to see how it was before)
  

Relevant test scenarios

• Changes should be tested with the browser console open
• Changes should be tested on different posts/pages/taxonomies/custom post types/custom taxonomies
• Changes should be tested on different editors (Default Block/Gutenberg/Classic/Elementor/other)
• Changes should be tested on different browsers
• Changes should be tested on multisite

Test instructions for QA when the code is in the RC

• QA should use the same steps as above.

QA can test this PR by following these steps:

Impact check

This PR affects the following parts of the plugin, which may require extra testing:

• No further impact

UI changes

• This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

Other environments

• This PR also affects Shopify. I have added a changelog entry starting with [shopify-seo], added test instructions for Shopify and attached the Shopify label to this PR.

Documentation

• I have written documentation for this change. For example, comments in the Relevant technical choices, comments in the code, documentation on Confluence / shared Google Drive / Yoast developer portal, or other.

Quality assurance

• I have tested this code to the best of my abilities.
• During testing, I had activated all plugins that Yoast SEO provides integrations for.
• I have added unit tests to verify the code works as intended.
• If any part of the code is behind a feature flag, my test instructions also cover cases where the feature flag is switched off.
• I have written this PR in accordance with my team's definition of done.
• I have checked that the base branch is correctly set.

Innovation

• No innovation project is applicable for this PR.
• This PR falls under an innovation project. I have attached the innovation label.
• I have added my hours to the WBSO document.

Fixes https://github.com/Yoast/plugins-automated-testing/issues/1993